This invention relates to the coating of a metal surface. More particularly, this invention relates to a method for coating a metal surface with a particulate mixture of a flux and a coating material comprising one or more metals or metal alloys in particulate form capable of reacting with the metal surface to form a protective coating thereon.
It is known to treat the surface of a ferrous metal with a flux to permit subsequent coating of the ferrous metal with an aluminum or aluminum alloy coating. For example, Owen U.S. Pat. No. 2,963,384 describes a method for coating a ferrous metal article with an aluminum metal coating which comprises immersing the ferrous metal in a molten flux bath containing NaCl, KCl, Na.sub.3 AlF.sub.6, K.sub.3 AlF.sub.6 and optionally AlF.sub.3 and NaF, and then immediately transferring the fluxed ferrous metal object into a molten bath of aluminum, or aluminum alloy.
Teshima et al U.S. Pat. No. 3,027,269 discloses a process for coating a ferrous metal base with aluminum which comprises applying to the ferrous metal base an aqueous flux solution containing alkali metal halides and/or alkali metal hydroxide and/or alkali metal azide, drying the coated ferrous metal base at a temperature over 400.degree. C., then heating the coated ferrous base in a reducing atmosphere at a temperature over 600.degree. C., and then dipping the coated ferrous metal base into molten aluminum.
Shoemaker U.S. Pat. No. 3,860,438 discloses a process for coating a ferrous metal with an alloy containing from 25 to 70 wt. % aluminum and the balance zinc which comprises wetting the ferrous metal with an aqueous flux consisting essentially of potassium fluosilicate, potassium fluoride, zinc chloride, and hydrofluoric or a mixture of hydrofluoric and hydrochloric acid, drying the coated ferrous metal, and then immersing it in a molten bath of the aluminum zinc coating alloy.
It is also known to flux an aluminum body to facilitate brazing aluminum pieces together. For example, Chartet U.S. Pat. No. 3,667,111 describes a process for fluxing and brazing metal parts of aluminum or aluminum alloy which comprises forming a brazing flux such as alkali metal chlorides or fluorides, cryolite, aluminum fluoride, and zinc chloride and depositing the flux, e.g., by spraying, at a temperature of from 200.degree.-400.degree. C., on the portions to be brazed together, either by heating the flux or the parts. The parts are then heated to the brazing temperature.
Aoki U.S. Pat. No. 4,571,352 teaches a method for coating an aluminum metal body with an aluminum alloy brazing filler material and a flux by melting an aluminum alloy brazing filler material in a melting tank and then floating a flux over the molten metal consisting of a potassium fluoaluminate mixture of K.sub.3 AlF.sub.6 and KAlF.sub.4. An aluminum metal body is then dipped into the molten aluminum alloy brazing filler material through the flux layer and then withdrawn from the molten brazing metal layer through the flux layer to coat the aluminum body with both the flux and the aluminum alloy brazing filler material.
Fluxes have also been used to form a contact between a metal and a nonmetal such as graphite or a ceramic. For example, Anderson U.S. Pat. No. 3,119,171 teaches a method for making a low resistance contact of indium on a graphite body which comprises applying a cesium fluoride flux to the graphite body and then heating the graphite in contact with indium or an indium alloy to a temperature in excess of 1100.degree. C.
Hodgkins U.S. Pat. No. 4,612,600 describes a process for sintering a base metal electrode such as a copper, manganese, cobalt, iron, or nickel electrode to a metal titanate ceramic such as barium titanate, calcium titanate, or strontium titanate using lithium fluoride or materials capable of forming lithium fluoride during sintering as a flux. The flux material and the ceramic materials are mixed together and formed into a sheet which is then coated with the base metal electrode. The coated sheet is then sintered at a temperature less than 950.degree. C.
In the prior art described, various methods are disclosed for applying the flux to the surface to be coated. For example, the aforesaid Owen patent immerses the metal to be coated into a molten flux while the Aoki patent immerses the metal to be coated in a molten bath of brazing filler metal on which the flux floats as an upper layer. The aforementioned Chartet patent deposits the flux on the metal to be coated as a spray at an elevated temperature.
The Teshima et al, Anderson, and Shoemaker patents discussed above apply the flux to a metal surface from an aqueous solution. Kozono U.S. Pat. No. 4,774,106 sprays a liquid such as water to the parts to be joined together to form a liquid film on the surface of the parts and then applies a flux in particulate form onto the water film.
The Hodgkins patent mixes the flux with the ceramic powders comprising the ceramic substrate prior to sintering of the ceramic with the metal electrode coated thereon.
However, it would be desirable to form a protective coating on a metal surface, without the need for a separate step to independently apply a flux to the metal surface. This would be particularly advantageous when forming a protective metal coating on a surface of a metal which could otherwise form oxides on the surface thereof after being treated with an oxide-removing flux agent and prior to having a protective metal coating formed thereon in a separate step.